The shift comes as cryptocurrencies—and especially "initial coin offerings" of new digital tokens—have come under increasing scrutiny by regulators. Last week, the Securities and Exchange Commission warned that many cryptocurrency exchanges are operating outside the law. Authorities have alleged that some cryptocurrency offerings are outright frauds, while many others are just extraordinarily risky investments.

Cryptocurrency markets reacted negatively to the news. The price of bitcoin is down about 6 percent since the Wall Street Journal first reported the new policy early Wednesday morning. Other major currencies, including Ethereum and Litecoin, are also down modestly.

Google's new policy is broad. The company is not only banning advertisement of cryptocurrencies themselves but also "initial coin offerings, cryptocurrency exchanges, cryptocurrency wallets, and cryptocurrency trading advice."

In the same announcement, Google said it is also banning binary options, a financial instrument that either pays out a fixed amount (like a dollar figure or a share of stock) or else pays nothing. This type of asset is prone to fraud, and some jurisdictions ban them outright.